1. Field of the Invention
The present invention relates to an electrostatic chuck. More specifically, the invention relates to an electrostatic chuck which is preferably used for conveying or holding the wafers in executing a variety of treatments in the steps of producing semiconductor devices, such as etching, CVD, sputtering and the like.
2. Description of the Related Art
In the steps of producing semiconductor devices inclusive of liquid crystal, it is necessary to hold the substrate such as a wafer to treat the substrate by etching or by exposure to light, or to convey the substrate. In particular, the electrostatic chuck for electrostatically holding the substrate can be used in vacuum or in a corrosive gaseous atmosphere, is suited for the production of semiconductor devices and is, hence, widely used.
In the plasma treatment apparatus which executes the treatment in vacuum, however, space between the wafer and the surface of the electrostatic chuck on where the wafer is placed becomes vacuum, and the heat is poorly transmitted between the wafer and the electrostatic chuck. Due to the irradiation with ions during the treatment with plasma, therefore, the water is locally heated and the temperature in the wafer becomes uneven resulting in the occurrence of defects.
Japanese Unexamined Patent Publication (Kokai) No. 2-119131 discloses that many grooves are formed in the wafer-placing surface of the electrostatic chuck in a radiating manner from the center toward the outer periphery thereof, a gas which is a medium for conducting the heat is introduced into the grooves to prevent the rise of temperature of the wafer and to decrease a dispersion in the temperature in the surface of the wafer.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-112302 discloses that the wafer-placing surface of the electrostatic chuck has a roughness which is not larger than 0.3 xcexcm to increase the contact area between the wafer and the electrostatic chuck, and to improve the conduction of heat in an attempt to evenly heat the substrate.
Japanese Unexamined Patent Publication (Kokai) No. 8-55905 discloses an electrostatic chuck in which the wafer-placing surface is divided by the a gas injection groove into a central region of the placing surface and an outer peripheral region of the placing surface, the outer peripheral region of the placing surface having a surface roughness of from 0.2 to 0.5 xcexcm, which is finer than that of the central region of the placing surface, in order to make the temperature distribution uniform on the surface of the water.
In the electrostatic chuck having a number of grooves radially formed from the central portion disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-119131, however, the heat is evenly distributed on the central portion where the grooves are densely arranged but the temperature fluctuates depending upon the locations since the gap among the grooves increases toward the peripheral portion, involving a problem of insufficient evenness in the distribution of heat.
In the electrostatic chuck taught in Japanese Unexamined Patent Publication (Kokai) No. 6-112302, the wafer-placing surface has a fine surface roughness enabling the substrate to be evenly heated. Due to its too large adsorptive force, however, there remains residual adsorption when the application of voltage is discontinued. Therefore, the response for separting the work becomes poor causing the throughput and the productivity to be decreased.
The electrostatic chuck disclosed in Japanese Unexamined Patent Publication (Kokai.) No. 8-55905 has a problem of an increased percent defective in addition to the above-mentioned problem of separation response. That is, the electrostatic chuck offers an improved evenness of temperature distribution due to a fine roughness in the surface of the electrostatic chuck. However, an increase in the temperature causes the wafer to be warped permitting the gas to leak from the gas injection grooves which are provided with gas holes near the circumferential edge of the substrate, adversely affecting the homogeneity in the treatment with plasma and the reproduceability, driving up the percent defective.
As described above, the conventional electrostatic chucks are not capable of satisfying all of the properties such as evenness in the heat distribution, adsorptive force and separation response causing, therefore, a decrease in the throughput in the steps of producing semiconductor devices and driving up the percent defective.
It is therefore an object of the present invention to provide an electrostatic chuck which features a sharp substrate separation response and a little gas leakage yet maintaining evenness in the heat distribution of the substrate and a large adsorptive force.
The present invention is based on a discovery that the substrate separation response and the prevention of gas leakage can be improved while maintaining evenness in the heat distribution of the substrate and a large adsorptive force by controlling the surface roughness and the height of the outer peripheral region and the central region in the placing surface.
According to the present invention, there is provided an electrostatic chuck comprising a ceramic dielectric layer having a placing surface for placing thereon a work that is to be held, and an electrode provided on a surface opposite to the placing surface of the ceramic dielectric layer for placing the work thereon or provided in the dielectric layer, wherein:
the placing surface of the ceramic dielectric layer is sectionalized into an outer peripheral region and a central region by gas injection grooved extending in a circumferential manner;
the surface roughness Ra(o) of the outer peripheral region and the surface roughness Ra(i) of the central region satisfy the following conditions:
0.6xe2x89xa6Ra(i)xe2x89xa61.5 xcexcm
Ra(o)xe2x89xa60.7 xcexcm
Ra(i)xe2x89xa7Ra(o)
and
the outer peripheral region is higher than the inner peripheral region by not less than 0.6 xcexcm.
As described above, the surface roughness Ra(i) in the central region of the placing surface is selected to be as large as 0.6 to 1.5 xcexcm, and the horizontal level is differed by not less than 0.6 xcexcm in height between the outer peripheral region of the placing surface and the central region of the placing surface, permitting the gas to enter into a gap between the work being held and the central region of the placing surface and improving evenness in the distribution of heat.
Owing to the difference in the height, the end of the wafer does not separate away from the placing surface even when the work to be held such as wafer (hereinafter simply referred to as wafer) is warped due to a rise in the temperature, since the deformation is absorbed. Accordingly, the leakage of gas from the outer peripheral region of the placing surface is prevented. Besides, since the surface roughness Ra(o) is finer than 0.7 xcexcm in the outer peripheral region of the placing surface, a large adsorptive force is obtained on the outer peripheral region of the placing surface.
Further, based on a discovery that the roughness in the placing surface greatly affects the adsorptive force and the residual adsorptive force, the inventors have rendered the outer peripheral region of the placing surface to possess a fine surface roughness and the central region of the placing surface to possess a relatively large surface roughness, in order to realize a sufficiently large adsorptive force and a quick separation response. That is, since the central region of the placing surface has a large surface roughness and the outer peripheral region of the placing surface has a fine surface roughness, the contact area is decreased between the wafer and the electrostatic chuck. Accordingly, a large adsorptive force is produced on the outer peripheral region of the placing surface but the adsorptive force is suppressed in the central region of the placing surface, and the work separation response is improved.
In particular, it is desired that the outer peripheral region of the placing surface and the central region of the placing surface have maximum surface roughnesses Rmax of not larger than 2 xcexcm. This makes it possible to prevent local leakage of gas and to improve evenness in the heat distribution of the work that is held, adsorptive force and separation response.
It is desired that the distance from the peripheral edge of the placing surface to the outer periphery of the gas injection groove is not larger than 10 xcexcm. This makes it possible to narrow the outer peripheral region of the placing surface, to widen the central region of the placing surface, to widen the region where the wafer is evenly heated by gas, to narrow dead space in the outer peripheral region, thereby to increase the number of the semiconductor chips obtained from a piece of substrate and to improve the productivity.
It is further desired that the sintered body forming the ceramic dielectric layer has a relative density of not lower than 98% and a maximum porous diameter of not larger than 2 xcexcm. This makes it possible to easily control the surface roughness of the placing surface and to decrease the amount of particles generated by the friction with the work to be treated.
It is further desired that the sintered body forming the ceramic dielectric layer has a volume resistivity of from 107 to 1012 xcexa9cm. This makes it possible to obtain a large adsorptive force based on the Jonsen-Rahbeck force with a low application voltage.
It is further desired that the ceramic dielectric layer is formed of a sintered body comprising aluminum nitride as a chief component. This makes it possible to obtain a product which is not corroded by a corrosive gas containing fluorine or chlorine or is not corroded by a plasma, and features extended life.